[TowerTalk] Broadside Parasitic Element

[Brian Beezley]

http://ham-radio.com/k6sti/broad.htm

The writeup describes a simple upgrade for an FM broadcast receiving 
antenna, but the idea may have application at HF. For example, if you 
hang a quarter-wave wire from a tree 90 feet away from a quarter-wave 
40-meter vertical, you'll get 3 dB gain in two directions. The wire will 
need radials or a good ground connection, but it will not need to be fed 
nor must the vertical be rematched. The half-wave 40-meter wire vertical 
I once used in a tall Eucalyptus tree would need only a parallel wire.

Due to the height needed when used with a horizontally polarized 
antenna, I think a parasitic broadside element is best applied to 
vertical antennas at HF.

A director can provide higher gain, but the array will have low 
impedance, require antenna rematching, have restricted bandwidth, and 
may have higher ground losses. A detuned reflector can provide about the 
same gain as a broadside parasitic element when placed 46 feet away from 
a 40-meter vertical. Impedance is high enough that you can get away 
without rematching. Response is unidirectional with low F/B. This gives 
another antenna upgrade option, depending on available supports.

A broadside parasitic element or a detuned reflector are quick and easy 
antenna upgrades. They're not something you'd design from scratch. 
Feeding both broadside elements will yield higher gain, as will properly 
tuning and matching an end-fire parasitic array.

In 30 years of antenna modeling, this is only the second design that has 
surprised me. The first is here: http://ham-radio.com/k6sti/tilted.htm. 
I stumbled across both while modeling something else. I like to pass 
along unexpected results. You never know when an antenna trick may prove 
useful.

I subscribed to Towertalk only to post this message. I'll unsubscribe 
when I see it appear. If you have a question, contact me by email at the 
address given in the writeup.

Brian